There are things we weren't meant to know about our world. Things lurk in the places between what is seen and unseen. They started noticing us, as we started noticing them.
As we kept building our steel and concrete empires ever higher, we began infringing upon their homes. Though we could not see them, they could see us. They knew of our arrogance, our need to constantly be the greatest of the living species.
They were content to let us pursue our need for greatness, so far as it didn't affect their own existence. Maybe if they had made themselves known earlier, we might have curbed our desires for expansion. Or maybe war would have broken out.
They did make themselves known eventually though, descending from the heavens inchingly, devouring our monuments and homes. There was worldwide panic. People screamed in the streets for weeks, militaries were scrambled.
Until they got to the surface. Then, then it was all quiet.